The present invention provides improved polymer electrolyte membranes for fuel cell applications and methods for producing the same and, in particular, to polymer electrolyte membranes for fuel cells. The membranes restrain crossover phenomenon of fuel and decomposition reaction of polymer membranes over platinum. Further, the membranes show outstanding power output and performance characteristics during operation of a fuel cell. A method for producing the membranes is also provided.
In our time, energy is a requisite and its importance continues to increase. Energy has been mostly obtained from fossil fuels, nuclear power generation, and water power generation. However, recently, many efforts have been made to develop technologies for effectively utilizing limited energy sources and exploiting various alternative energy sources because of depletion of natural energy sources and increase of environmental concerns. In addition, advanced countries have focused on these technologies to take the initiative in the future of the energy industry.
A fuel cell meeting the above needs is one of the future energy sources now in the spotlight. The fuel cell is a kind of D.C. generator (direct current generator) directly transforming chemical energy into electrical energy by an electrode reaction, and has a high energy efficiency. The fuel cell is not limited by a Carnot cycle, as well as hardly causing problems of noise, vibration, and waste gas. In addition, the fuel cell is advantageous in that the fuel cell can continuously generate electric power if fuel and oxidants are continuously provided, while first and secondary cells store a limited supply of energy and for its operation. Recently, there has been actively studied a fuel cell as a high efficiency energy source in USA, Europe, Canada, and Japan, and it is expected that the fuel cell will soon be commercialized as an effective alternative energy source.
Fuel cells are classified into various groups including polymer electrolyte fuel cells (PEFC) or proton exchange membrane fuel cells (PEMFC), alkali fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), and solid oxide fuel cells (SOFC). The classifications are usually made according to operating temperatures and classes of electrolytes.
The various types of fuel cells and components employed therein are well known in the art. For example, U.S. Pat. No. 5,798,188 to Mukohyama et al is directed to a polymer electrolyte membrane fuel cell with bi-polar plate having molded polymer projections. U.S. Pat. No. 6,383,676 to Akiyama et al is directed to polymer electrolyte fuel cell devices with features that prevent the polymer electrolyte membrane from drying. U.S. Pat. No. 5,360,679 to Buswell et al is directed to hydrocarbon-fueled solid polymer fuel cell electric power generation systems to produce utility-grade electrical power. U.S. Pat. No. 6,387,558 to Mizuno et al is directed to fuel cells with separators for the distribution of gas in the fuel cell. U.S. Pat. No. 5,547,551 to Bahar et al is directed to ultra-thin integral composite membranes for fuel cells. U.S. Pat. No. 6,391,486 to Narayanan et al is directed to an improved direct liquid-feed fuel cell having a solid membrane electrolyte. These references are incorporated herein in their entirety and provide background for the various types and components of fuel cells.
Among these various types of fuel cells, polymer electrolyte fuel cells mostly use a Nafion® polymer membrane having hydrogen ion conductive property as an electrolyte. The advantages of PEFCs include low operation temperature, high efficiency, high currency density and output density, short starting time, and fast response to load change, in comparison with other types of fuel cell. In addition, the polymer electrolyte fuel cell is not eroded and strict control of the electrolyte is not needed because the polymer membrane is used as the electrolyte. Further, conventional established technology utilizing a methanol reformer can be applied to this type of fuel cell. Other advantages of a polymer electrolyte fuel cell are that it is not sensitive to pressure changes of reacting gas, it can be easily produced because its structure is simple, and a fuel cell stack can be made of various materials. Furthermore, volume and weight of the polymer electrolyte fuel cell are smaller than the phosphoric acid fuel cell having the same operating principle as the polymer electrolyte fuel cell, and the polymer electrolyte fuel cell can be used in applications such as a power source of a nonpolluting car, on-site generation of electricity, an electricity source of a spacecraft, a portable energy source, and an energy source for military purposes, because it can generate various ranges of power.
However, the polymer electrolyte fuel cell is disadvantageous in that waste heat cannot be utilized and the polymer electrolyte fuel cell cannot be used in conjunction with a reformer operated at high temperature. Because the polymer electrolyte fuel cell is operated at low temperature, a maximum limit of carbon dioxide content in reaction gas is low because platinum is used as an electrode catalyst. Catalyst content should be very low in order to reduce production cost of the polymer electrolyte fuel cell. Particularly, a nafion polymer membrane used as the electrolyte is very expensive, and it is difficult to control moisture content in the polymer membrane during operation of the fuel cell.
In the case of a polymer electrolyte fuel cell using a polymer membrane as the electrolyte, the contact surface between the electrolyte and a catalyst in the electrode is small in comparison with other fuel cells using a liquid electrolyte, thereby a large quantity of catalyst is needed.
The nafion polymer membrane is a kind of perfluorinated ionomer membrane, taking the shape of a transparent film with a thickness of about 150 μm. It has an equivalent weight of about 1100, and when it is hydrated, it has a high hydrogen ionic conductivity of 10−2 S/cm or higher. However, the nafion polymer membrane is relatively thick, and so an output characteristic of the polymer electrolyte fuel cell is poor during operation of the fuel cell, a crossover problem of fuel occurs when methanol is used as fuel, and the ionic conductivity of the nafion polymer membrane is sensitive to temperature and relative humidity. In addition, when the nafion polymer membrane is used at a relatively high temperature, it is thermally deformed, and so the nafion polymer membrane does not come into sufficient contact with the electrode, thereby performance of the polymer electrolyte fuel cell is seriously reduced.